Our goal is to determine if the dietary n-3 class of polyunsaturated fatty acids (PUFAs), provided in our diet largely from marine fish oils, will prevent fatal ventricular tachycardia (VT) and fibrillation (VF) in humans. Since there are some 250,000 deaths annually in the USA alone from sudden cardiac death within one hour of a heart attack, nearly all of which are due to VT or VF, this is a major public health problem for which at the present time there is no specific, safe, or effective preventive or therapy. It is often the first indication of serious heart disease. The suggestive clinical and our definitive experimental evidence for an antiarrhythmic effect of n-3 PUFAs urgently support a rigorous clinical trial now. Aim: To determine if regular ingestion of n-3 PUFAs will significantly reduce the risk of sudden cardiac death. We propose to conduct a prospective, placebo-controlled, double blinded, clinical trial in patients with implanted cardiac defibrillators (ICDs) to test whether the regular ingestion of a supplement of a fishoil will significantly reduce the number of therapeutic discharges of the ICDs over a period of one year, as compared with an olive oil supplement, which we have found has no antiarrhythmic action. This will be a secondary prevention trial in 400 patients who have had ICDs implanted for a prior episode of cardiac arrest, from which they were fortunately resuscitated, with no need to alter usual care or risk to the patient. We will only need to record from the ICDs the number of therapeutic discharges and confirm from the recorded electrocardiogram strips the kinds of arrhythmias which had induced the ICD response. This appears to be the simplest, safest, quickest, least expensive, non invasive, definitive clinical trial to test our hypothesis. It is a true mortality end-point trial as every ICD discharge would be a death if the subjects were not protected by the ICD. We have been developing the protocol for this study for the past three years while establishing the evidence in the laboratory that these n-3 PUFAs are potent antiarrhythmic agents.